1. Field of the Invention
The present invention relates to an inverter controller for driving a motor, which uses a small-capacity reactor and a small-capacity capacitor, and also relates to an air conditioner using such an inverter controller as an inverter apparatus.
2. Description of the Related Art
As a general inverter controller for driving an induction motor used in an all-purpose inverter and the like, a V/F control type of inverter controller for driving an induction motor shown in FIG. 20 is well-known as disclosed, for example, in a non-patent document 1 (see page 661 to 711 in “Inverter Drive Handbook”, compiled by an editing committee of the Drive Handbook, first edition in 1995, published by THE NIKKAN KOGYO SHIMBUN, LTD, for example).
Referring to FIG. 20, a main circuit includes a DC power supply apparatus 203, an inverter 3 and an induction motor 4. The DC power supply apparatus 203 includes an AC power supply 1, a rectifier 2, a smoothing capacitor 202 storing electric energy for a DC voltage source of the inverter 3, and a reactor 201 for improving a power factor of the AC power supply 1.
Meanwhile, a control circuit includes a V/F control pattern section 7, a motor voltage command generator 8 and a PWM controller 12. The V/F control pattern section 7 is provided for deciding a motor voltage value applied to the induction motor 4 based on a speed command ω* of the induction motor 4 applied from the outside. The motor voltage command generator 8 is provided for generating a motor voltage command value of the induction motor 4 based on the motor voltage value decided by the V/F control pattern section 7. The PWM controller 12 is provided for generating a PWM signal of the inverter 3 based on the motor voltage command value generated by the motor voltage command generator 8.
FIG. 21 shows an example of the general V/F control pattern generated by the V/F control pattern section 7. As shown in FIG. 21, it is constituted such that the motor voltage value applied to the induction motor 4 is unambiguously decided in relation to the speed command ω*. In general, the speed command ω* and the motor voltage value are stored in a memory of a calculating device such as a microcomputer as table values, and the motor voltage value is provided by liner interpolation from the table values for the other speed command ω* which is not included in the table values.
Here, when the AC power supply 1 is 220 V (AC power supply frequency is 50 Hz), the input of the inverter 3 is 1.5 kW, and the smoothing capacitor 202 is 1500 μF, a relation between a harmonic component of an AC power supply current and an order to an AC power supply frequency when the reactor 201 for improving the power factor is 5 mH and 20 nH is shown in FIG. 22.
FIG. 22 shows the relation together with the IEC (International Electrotechnical Commission) standard, from which it is seen that the third harmonic component especially largely exceeds that of the IEC standard when the reactor 201 for improving the power factor is 5 mH. Meanwhile, the IEC standard is satisfied until the fortieth harmonic component in the case of 20 mH.
Thus, it is necessary to take measure to further increase an inductance value of the reactor 201 for improving the power factor in order to clear the IEC standard at the time of especially high loading, and it causes the inverter to increase in size and weight, which increases cost.
Therefore, there is proposed a DC power supply apparatus shown in FIG. 23, for example, in a patent document 1 (Japanese Patent Unexamined Laid-open Publication H9-266674). Referring to FIG. 14, a DC power supply apparatus is improved to prevent an increase of the inductance value of the reactor 201 to increase the power factor, while reducing the power supply harmonic component and increasing the power factor.
In FIG. 23, an AC power supply voltage of an AC power supply 1 is applied to an AC input terminal of a full-wave rectifier constructed by bridge connection of diodes D1 through D4, the output thereof is charged into a middle capacitor C through a reactor Lin, the charges of the middle capacitor C is discharged to a smoothing capacitor CD and a DC voltage is supplied to a load resistance RL. In this constitution, a transistor Q1 is connected in a negative and positive DC current path connecting a loading side of the reactor Lin to the middle capacitor C, and this transistor Q1 is driven by a base driving circuit G1.
In addition, pulse generation circuits I1 and I2 applying a pulse voltage to the base driving circuit G1, and a dummy resistance Rdm are further provided. Each of the pulse generation circuits I1 and I2 comprises a circuit for detecting a zero cross point of the AC power supply voltage and a pulse current circuit for applying a pulse current to the dummy resistance Rdm after the zero cross point is detected until an instantaneous value of the AC power supply voltage becomes equal to a voltage at both ends of the middle capacitor C.
Here, the pulse generation circuit I1 generates a pulse voltage in the former half of a half-cycle of the AC power supply voltage, and the pulse generation circuit I2 generates a pulse voltage in the latter half of the half-cycle of the AC power supply voltage.
In addition, when a current is forced to flow in the reactor Lin by turning the transistor Q1 on, a diode D5 for backflow prevention is connected such that the charge in the middle capacitor C may not be discharged through the transistor Q1, and a diode D6 for backflow prevention and a reactor Ldc for increasing a smoothing effect are connected in series in a path in which the charge in the middle capacitor C is discharged to the smoothing capacitor CD.
In the above constitution, the transistor Q1 is turned on at a part or a whole of a phase section in which the instantaneous value of the AC power supply voltage does not exceed the voltage at both ends of the middle capacitor C, and thus the harmonic component is reduced and a high power factor is attained without making the apparatus large.
However, in the above conventional constitution as disclosed in the patent document 1 which describes a simulation result in the case of 1500 μF and 6.2 mH, the smoothing capacitor CD and the reactor Lin having large capacity are still provided, and the middle capacitor C, the transistor Q1, the base driving circuit G1, the pulse generation circuits I1 and I2, the dummy resistance Rdm, the diodes D5 and D6 for backflow prevention, and the rector Ldc enhancing the smoothing effect are further provided. Therefore, the apparatus becomes large in size and its cost is increased because the number of parts is increased.